This invention relates to spray drying systems wherein the material to be dried is introduced into the exhaust stream of one or more pulse jet engines. More particularly, this invention relates to a material injection nozzle for use in such a drying system to thereby adapt the drying system for the processing of a variety of materials.
Various spray drying systems wherein pulse jet engines provide hot gaseous flow and broadband acoustic energy for removing or reducing the moisture content of solutions and liquids that contain suspended particulate matter are known to the art. For example, Lockwood, U.S. Pat. No. 3,618,655, and E. E. Anderson, U.S. Pat. No. 3,586,515, (both of which are assigned to the assignee of this invention) disclose pulse jet drying systems that are primarily addressed to dehydrating fish that is pulverized into a slurry to obtain fishmeal. Additionally, the copending patent application of Frederick Ferguson (assigned to the assignee of this invention and filed of even date with this application) discloses a pulse jet drying system which is configured to provide increased thermal efficiency and the high capacity processing of a wide variety of materials.
The pulse jet drying systems disclosed in the above noted patents and copending patent application are basically characterized in that the liquid containing the material to be removed or reclaimed is introduced into the exhaust pipe of one or more pulse jet engines at a predetermined distance from the exhaust pipe exit opening and is borne upwardly through openings in the floor of a vertically extending tank or chamber. The pulsating hot gaseous exhaust stream and the attendant high level acoustic energy atomizes or separates the injected material into a spray so that substantial evaporation occurs as the material travels along the pulse jet engine exhaust pipe. Further, each of the above mentioned systems is arranged to induce air currents which circulate upwardly and about the interior of the drying chamber to further dry the material being processed.
One of the disadvantages and drawbacks of prior art pulse jet drying systems such as the system disclosed in the previously mentioned patents to Lockwood and Anderson is that the system is not suitable for use in the drying of certain substances. In particular, in these systems the substance to be dried is introduced into the exhaust pipe of a pulse jet engine by means of a feedpipe that passes coaxially along the interior pulse jet engine exhaust pipe with the terminating end of the feedpipe being of the same geometry and size as the major portion of the feedpipe or, alternatively, being flared outwardly. In attempting to process liquids containing rather fine particles of organic or other temperature sensitive material, it has been found that partially dried particulate matter travelling through the pulse jet engine exhaust pipe adheres to the wall of the exhaust pipe. Because the exhaust pipe and the surrounding region of the drying chamber floor are hot, the adhering material rapidly forms a burned, hardened layer that continues to build up as the system operates. The buildup of such material not only causes smoke and cinders that can affect the quality of the dried material, but, more importantly, directly affects the operation of the pulse jet drying system. Specifically, as the material accumulates about the exit region of the exhaust pipe, the resonant operation of the pulse jet engine is affected and, when a sufficient amount of material has accumulated, the engine will cease to operate. When this occurs, the entire drying system must be shut down and the accumulated, burned material must be scraped and chipped away.
For example, in experimental tests for processing waste brewing liquids to reclaim yeast in which the feedpipe was configured according to the teachings of the Lockwood patent and terminated approximately 181/2 inches below the pulse jet engine exhaust pipe exit opening, heavy buildup occured which caused the pulse jet engine to cease functioning after a very short processing interval. More specifically, in one such test the pulse jet drying system was operated for an initialization period of 40 minutes during which the yeast containing fluid was introduced into the pulse jet engine exhaust pipe at a rate of 1.875 gallons per minute. The material feed rate was then increased to approximately 2.625 gallons per minute and, after only slightly more than ten minutes of additional system operation, the pulse jet engine quit functioning. When the drying system was shut down and inspected it was determined that the uppermost 8 to 10 inches of the pulse jet engine exhaust pipe was heavily encrusted with burned yeast.
Considering the fact that even the more concentrated yeast solutions of conventional brewing processes have a yeast content of only about 30% and a weight of about 8.7 lbs. per gallon, it can be seen that the prior art material injection arrangement is not satisfactory for commercially reclaiming brewer's yeast. In particular, even if the prior art arrangement would operate over a sufficient time interval, a feed rate of 1.875 gallons per minute of the above mentioned 30% solid material provides approximately 300 lbs. per hour of dried brewer's yeast having a moisture content of approxximately 7% (per engine). Further, since yeast solutions which typically result from brewing processes do not typically include 30% solid material, but often have a solid content on the order of 10 to 20%, reclamation of the yeast at such low feed rates becomes even less attractive. Since systems of the type disclosed in the previously mentioned Lockwood patent typically operate with thermally efficiencies on the order of 50 to 60% and more efficient systems such as the system disclosed in the previously referenced copending application of Frederick Ferguson can attain drying efficiencies ranging between 80 and 95%, it can be recognized from the above mentioned experimental tests that the prior art arrangement for material injection not only would fail to provide satisfactory drying rates, but would be extremely wasteful of energy resources in drying heat sensitive, fine particulate matter such as brewer's yeast.
Accordingly, it is an object of this invention to provide a material injection nozzle which eliminates or greatly reduces the accumulation of burned material within and around the exhaust exit of the pulse jet engines utilized in a spray drying system.
It is another object of this invention to provide a material injection nozzle for the pulse jet engine of a spray drying system which permits high capacity drying of heat sensitive materials without requiring system interruption at short intervals to scrape and clean the system.
It is yet another and attendant object of this invention to provide an improved pulse jet drying system that is adapted for high capacity processing of liquids containing heat sensitive particulate matter.